Method of bolt hole strengthening in a fibrous composite laminate

ABSTRACT

A method of strengthening a bolt hole in a fibrous composite laminate when a hole is drilled in a laminate sheet. The method allows for increased thickness strength, bearing strength and the use of low cost high strength fasteners which may or may not be galvanically compatible through the use of plating the edges of the holes with a metal spray.

BACKGROUND OF THE INVENTION

This invention relates to a method of strengthening of a bolt hole andmore particularly, but not by way of limitation to a method ofstrengthening the bolt hole in a fibrous composite laminate.

Heretofore, in the design of fibrous composite parts, ply padups havebeen required when holes are drilled in laminate sheets. This extramaterial is required because of reduced allowables due to notchsensitivity of the laminate caused by interlaminar shear stresses aroundthe edges of the bolt holes. These interlaminar shear stresses are aresult of edge effects and delamination caused by the relatively lowmaterial properties of the "through-the-laminate" direction.

Current methods of suppression of edge effects around bolt holes arethrough the use of stitching of a B-staged laminate and providingquasi-isotropic padups in the area of the bolt holes. A limitation tothis stitching method is that the laminate can only be stitched whileB-staged. Once cured, the laminate cannot be altered. Providingquasi-isotropic padups in areas to be bolted works fairly well inincreasing laminate bearing strength, but the reduced longitudinalstiffness requires a substantial increase in laminate thickness which isdirectly related to increased weight.

Further, highly unidirectional composite laminate sheets exhibit poorbolt hole properties such as thickness strength, bearing strength andgalvanic corrosion associated with conventional aircraft fasteners.

In the following United States patent, U.S. Pat. Nos. 2,700,172 to Rohe,3,158,503 to Young, 3,526,072 to Campbell, 3,895,409 to Kwatonowski,3,977,146 to Wiley, 4,098,922 to Dinella et al, 4,118,855 to Lequeux,4,232,496 to Warkentin and 4,296,586 to Heurteux various types offastening devices and grommet assemblies are shown which are used withdifferent types of airframe structures and composite materials. None ofthese prior art patents point out the distinguishing features of thesubject method of bolt hole strengthening as described herein.

SUMMARY OF THE INVENTION

The subject method of bolt hole strengthening in a fibrous compositelaminate suppresses edge effects by plating the edges of a laminate boltholes with a metal compatible with the fiber such as titamium, aluminumand similar metals. The metal plating of the bolt hole provides highstrength in the "through the laminate" direction and provides increasedbearing strength. The increased strength of the bolt hole allowsincreased allowables and thus a substantial decrease in the overallweight of a composite aircraft.

The method of bolt hole strengthening of a fibrous composite laminateincludes the steps of drilling a hole through the laminate sheet andapplying a metal spray by spraying, soldering or electro-plating thesides of the bolt hole. The liquid metal wicks into delamination andfills voids naturally occuring on the edges of fibrous compositelaminate. The plating maybe in the range of a few mils thickness orgreater for providing a substantial increase in the "through thelaminate" strength.

The advantages and objects of the invention will become evident from thefollowing detailed description of the drawings when read in connectionwith the accompanying drawings which illustrate preferred embodiments ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the metal spraying of a bolt hole through a laminatesheet.

FIG. 2 illustrates an alternate embodiment of the invention by applyinga heated mandrel inside the bolt hole adjacent a metal liner having aB-stage adhesive around the pheriphery of the bolt hole.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1 a fibrous composite laminate sheet 10 is shown incross-section having a drilled hole 12 therethrough. A flame or plasmametal spray applicator 14 having an opening 16 therein is placed insidethe hole 12 for introducing a metal spray indicated by arrows 18 to theedges of sides 20 of the bolt hole 12. Also shown in this drawing is thesheet 10 having preloads around hole pheriphery thereon indicated byarrows 22. By preloading the sheet 10 prior to drilling the hole 12 thedelamination and voids around the sides 20 of the hole 12 is greatlyreduced.

A metal plating 24 of the bolt hole 12 can be accomplished by a numberof methods such as flame spraying, soldering, electro-plating andsimilar types of applications. The liquid metal 18 wicks intodelamination and voids naturally occuring along the edges of the sides20 of hole 12. The metal plating 24 or reinforcement may be in athickness in the range of 0.002 to 0.02 inches for providing asubstantial increase in the "through-the-laminate" strength.

In FIG. 2 an alternate embodiment of the method of the invention isshown using a heated mandrel 30 inserted in the hole 12 of the compositelaminate sheet 10. The sheet 10 is also under a preload indicated byarrows 22. In this embodiment a metal liner 32 is introduced with aB-stage adhesive 34 introduced between the liner 32 and the sides 20 ofthe hole 12. The heated mandrel 30 heats and expands the metal liner 32and through conduction, the adhesive 34 is heated and adheres to thesides 34 of the hole 12 for providing a bonded metal liner therein.

Changes may be made in the construction and arrangement of the parts orelements of the embodiments as described herein without departing fromthe spirit or scope of the invention defined in the following claims.

What is claimed is:
 1. A method of strengthening a bolt hole in afibrous composite laminate sheet, the steps including:drilling a holethrough a composite laminate sheet; applying an adhesive around thesides of the hole; inserting a metal liner inside the hole adjacent theadhesive; and applying a heated mandrel inside the hole and adjacent theinner circumference of the metal liner and heating and forming anadhesive bond between the liner and the sides of the hole.